Why are so many isekai BLs so freaking weird?

Idk abt straight isekai, but SO many of them start w them as a child and grow up w the ml. I understand starting off as a kid, so you get a sense of familiarity and belonging to the world as you grow as an adult. BUT WHY DOES THE ML HAVE TO STICK AROUND THE WHOLE TIME??? Bro ur telling me you can just become sexually attracted to someone the moment they become legal? Like flipping a fucking switch? Clearly that’s not the case, cuz there are times when the mc gets charmed by the kid. It doesn’t matter that the younger one advances at the older first, as a GROWN adult, you can set clear boundaries. Like why can’t the ml show up once the both of them are adults? Or better yet, have them not isekai into a child’s body? A lot of these BL authors are just so fucked in the head to even think of plots like these.